An improved mechanical shutter is described which has applications in optical systems designed for point or line-imaging measurements. The shutter described has a fairly short exposure time (xcx9c9 xcexcs), 100% throughput, and essentially zero leakage in the closed state.
The shutter described herein is particularly well suited for spectroscopic applications, in which it replaces the front entrance slit of a spectrograph. Implementation of this shutter permits the use of non-intensified detectors for spectrographic applications in which continuous background radiation must be significantly reduced. Non-intensified detectors have better quantum efficiency, linearity, and dynamic range than their intensified counterparts.
It is well known that photographic exposure of a photographic material is achieved by the combined effect of image intensity and the time this energy is allowed to act on the photographic material. Accurate control of the duration for which energy is allowed to act on the photographic material is therefore important. Shutter mechanisms are used to effect this control.
Shutter mechanisms can be categorized in accordance with their positioning within the camera, for example, front shutters, xe2x80x98between-lensxe2x80x99 shutters and focal plane shutters. Most modern cameras employ a type of xe2x80x98between-lensxe2x80x99 shutter.
In its simplest form, the xe2x80x98between-lensxe2x80x99 shutter is a sector shutter comprising a circular metal plate with an elongated aperture. The metal plate is positioned perpendicular to the light path and makes one rotation when operated. Speed is controlled by a simple spring which is usually tensioned by a part of a release lever. Sector shutters are inexpensive and are often fitted to cheap cameras where inconsistencies which develop over a period of time and the limited speed range are not important.
The most commonly used type of photographic shutter is the bladed or diaphragm shutter. This type of shutter comprises three to five thin metal blades which open and shut across the light path. The blades are designed to minimize the time taken for opening and closing during the exposure cycle. Each shutter blade is pivoted at its outer edge and linked to a ring which is rotated to open the blades. Power for this rotation comes from a main driving spring which is tensioned prior to the shutter being activated. Diaphragm shutters are compact and offer a wide range of speeds.
These types of mechanical diaphragm shutters, however, are limited to speeds above several hundred microseconds. Furthermore, their repetition rate is dependent upon the arming cycle on the tensioning spring. An apparatus using this type of shutter, therefore, is usually directed to xe2x80x9csingle-eventxe2x80x9d applications.
For continuous and repetitive observations where high time resolution is necessary the use of a xe2x80x9cchopperxe2x80x9d as a shutter to limit the time duration for light admittance into a camera is well known. An example is shown in U.S. Pat. Ser. No. 4,954,714 to Pollak, et al.
In comparison, competing technology, based on ferroelectric liquid crystal retardation plates, results in shutters with exposure times on the order of 40 xcexcs, only about 40% throughput, and approximately 0.03% leakage (due to the continuous background luminosity in flames, even small leakage can integrate to a large spurious signal).
Accordingly, it is an object of the invention to provide a spectrometer provided with an optical shutter which has overcome the problems residing in the prior art.
It is another object of the invention to provide a spectrometer provided with an optical shutter which is capable of increasing the switching speed of the optical shutter, thereby enabling a high speed response.
It is yet another object of this invention is to provide a shutter capable of exposure times on the order of 10 microseconds while simultaneously providing 100% light admittance while open and near 0% light admittance when closed.
It is yet another object of this invention to provide a shutter which provides for synchronization with a separate light source, or other peripheral device, and which provides a means for determining the actual exposure time at the operating conditions employed.
Additional objects and advantages will become apparent to those having skill in these arts as the description and claims are reviewed.